1. Field of the invention
The present invention relates to an organic electroluminescence (EL) display device, more particularly, to an organic EL display device comprising a sealant covering an organic light-emitting device and a moisture absorbing material formed on an upper substrate.
2. Description of Related Art
Generally, in an organic EL display device, electrons and holes are injected into an emitting layer from a cathode and an anode, respectively. The electrons and holes recombine in the emitting layer to form excitons, which emit light when they transition from an excited state to a ground state.
Since the organic EL display device is self emissive, unlike a liquid crystal display device, it does not require a separate light source. Therefore, it may be made thin and light-weight.
The organic EL display device may be a passive matrix (PM) type or an active matrix (AM) type according to its driving method.
Although the PM type organic EL display device may be simply fabricated, it consumes a lot of power, it is not easily formed as a large screen device, and as the number of wirings increases, its opening ratio decreases.
Therefore, the PM type organic EL display device may be used for small sized display devices, while the AM type may be used for a display device having large area.
However, a conventional organic EL display device may degrade since an organic light-emitting layer material and a cathode electrode material typically have low moisture and oxidation resistance, which may result in a non-emitting region (i.e. dark spot). Over time, the dark spot may spread to the point where the device no longer emits any light at all.
Therefore, in order to solve the foregoing problems, the organic EL display device may be encapsulated to prevent exposure to moisture and oxygen. Encapsulation typically entails bonding a lower insulating substrate to an upper substrate by means of a sealant.
The foregoing encapsulation technology includes a method for coating a sealant on the outer circumference of a region at which a light-emitting device is formed, and then bonding the light-emitting device to the encapsulation substrate by means of the sealant. However, the sealant's limited width constrains its ability to prevent infiltration of external moisture or oxygen.
Japanese Patent Publication No. 2001-338755 discloses an organic EL display device having a sealant comprising a first sealant and a second sealant coated on the front surface of a region at which the light-emitting device is formed, as illustrated in FIG. 1.
Referring to FIG. 1, an organic light-emitting device 110, which may comprise an organic emitting layer between an anode electrode and a cathode electrode, is formed on an insulating substrate 100, and the light-emitting device 110 is encapsulated with an encapsulation substrate 120 using a sealant 130 having a two layer structure of a first sealant 131 and a second sealant 135.
The first sealant 131 may be formed of a halide, an alkali metal or an alkaline earth metal, and the second sealant 135 may be formed of moistureproof material.
However, in such an organic EL display device, since the first sealant 131 contacts the organic light-emitting device, it may degrade the organic light-emitting device 110.